Field
The presently disclosed subject matter relates to an optical deflector. The optical deflector can be applied as an optical scanner to a laser projector, a laser radar, a bar code reader, an area sensor, a head lamp, a head-up display unit, and other optical apparatus, to generate scanning light.
Description of the Related Art
Generally, in an optical scanner or the like, an optical deflector is constructed by a micro electro mechanical system (MEMS) device manufactured by using semiconductor manufacturing processes and micro machine technology.
A prior art optical deflector as a MEMS device is constructed by a mirror, an inner frame surrounding the mirror, a pair of torsion bars coupled between the mirror and the inner frame along an axis (X-axis), inner piezoelectric actuators coupled between the torsion bars and supported by the inner frame via inner coupling portions, serving as cantilevers for rocking the mirror with respect to the X-axis of the mirror, and an outer frame surrounding the inner frame via outer coupling portions (see: JP2013-080068 & US2013/0083379A1). In the above-described prior art optical deflector, the inner frame is very thick to have a high rigidity to sufficiently support the inner piezoelectric actuators. For example, the thickness of the inner frame is three or more times that of the piezoelectric actuators.
In the above-described prior art optical deflector, however, the rigidity of the outer coupling portions between the inner frame and the outer frame is small as compared with that of the inner frame. As a result, the resonant energy of the mirror is consumed by the inner frame, and in addition, a part of the resonant energy is leaked via the outer coupling portions to the outer frame. This means that the energy of the mirror energized by the piezoelectric actuators at the resonant frequency cannot be concentrated on the mirror, and this energy is dispersed into a frequency region around the resonant frequency, so that the quality factor Q of the mirror and the torsion bars at the resonant frequency would be decreased.
The quality factor Q is defined byQ=fr/(fb−fa)
where fr is the resonant frequency;
fa is a frequency lower than the resonant frequency fr at which the vibration energy is a half value of the vibration energy at the resonant frequency fr; and
fb is a frequency higher than the resonant frequency fr at which the vibration energy is half the value of the vibration energy at the resonant frequency fr.
The decrease of the quality factor Q would increase the drive voltages at the piezoelectric actuators in order to realize a desired deflection angle of the mirror.